Control Strategies For Microgrid With Non-linear And Unbalanced Loads

This thesis was supported by Natural Science Foundation of Chongqing(CSTC2009BB6190) and independent research project of State Key Laboratory of PowerTransmission Equipment&System Security and New Technology(2007DA10512711209).Due to the increasing social demands for electricity, the supply shortages oftraditional energy sources, as well as the large area blackout caused by failures in recentyears, the drawbacks of the traditional power grid are gradually exposed. Theinstallation of distributed generators near local loads is an economic and environmentalscheme to solve this problem. Microgrid, as an independent controllable systemconsisted of the distributed generators, local loads and energy storage systems,effectively alleviates the adverse effects of distributed generation on the voltage, powerflow, and reliability of the distribution network. And it becomes a research hotspot inrecent years. The nonlinear and unbalanced load in microgrid will increase the voltageunbalanced factor, the total harmonic distortion, and debase the power quality. It is ofgreat significance to effectively inhibit transmission of the caused harmonics andunbalanced components in the microgrid, and improve the power quality of the PCCs(Point of common coupling) and ensure reliable, constant and high quality power supplyfor the various types of loads in microgrid. The main contents are as follows:①Proposes a compensation algorithm for microgrid with nonlinear and unbalancedload in dq0reference frame though instantaneous power theory. Takes local signals asinputs and derivates compensation algorithm in dq0reference frame based on thederivation of reference compensation currents in abc reference frame. Based on thedifferent compensation requirements caused by the relative capacity of DGs and loadsand scheduling, the two compensation modes is proposed.②According to the different types of local loads, the control of parallel inverters isresearched. The droop control method is adopted for DGs with linear and balanced localloads. For DGs with nonlinear and unbalanced local loads the inverse droop control isintroduced, and according to the characteristics of microgrid the reference currentscalculation method in the islanded mode is revised. With variable width, the hysteresiscontrol is adopted to generate the switch signals of the power electronic devices.③Proposes a comprehensive control strategy for microgrid with nonlinear and unbalanced loads according to the different control methods of DGs and compensationdemand. The DGs with linear balanced loads adopts the droop control in both modesand the DGs with nonlinear and unbalanced loads combines the PQ control and thereverse droop control for the inverter control. Based on the two compensation modes,the independent and the collaborative control mode are proposed. Then the applicabilityof the control strategies in the microgrid with multiple DGs is discussed.④Builds a simulation model of microgrid with nonlinear and unbalanced loads onPSCAD/EMTDC platform. The comparative analysis of the compensation performancein grid-connected mode and islanded mode are conducted, and the dynamic responsecharacteristics of microgrid during modes switch are also studied. The correctness of theproposed compensation algorithm as well as the feasibility of the designed microgridcontrol strategy is verified. Simulation in different compensation modes and discuss ofthe compensation performance and its influence on microgrid are carried out.